linux_xanmod: force X86_AMD_PSTATE=y

https://github.com/NixOS/nixpkgs/pull/197295 enables it as a module by default
for recent kernel versions. We want it built-in on every xanmod version as they
are all patched to have the functionality.

Using mkImageMediaOverride to override the default kernel config but still allow
the user to mkForce it themselves.

Fixes https://github.com/NixOS/nixpkgs/issues/199498

Co-authored-by: Bernardo Meurer <bernardo@meurer.org>

pkgs/os-specific/linux/kernel/xanmod-kernels.nix

@@ -27,7 +27,7 @@ let
 
     structuredExtraConfig = with lib.kernel; {
       # AMD P-state driver
-      X86_AMD_PSTATE = yes;
+      X86_AMD_PSTATE = lib.mkOverride 60 yes;
 
       # Google's BBRv2 TCP congestion Control
       TCP_CONG_BBR2 = yes;